Bilateral coaxial resistive device



M. CONNEY BILATERAL COAXIAL RESISTIVE DEVICE March 28, 1967 2 Sheets-Sheet 1 Filed April 15, 1965 FIG.IA

B G F ATTOIP/Vf) March 28, 1967 M. CONNEY 3,311,855

BILATERAL COAXIAL RESISTIVE DEVICE Filed April 15, 1965 2 Sheets-Sheet 2 7a w FIG.2A

INVENTOR. MA/PCEALUJ CON/V5) BY JMW ATTOANEX United States Patent Office 3,311,855 BELATERAL CUAXHAI. RESESTIVE DEVICE Marceilus Cortney, 13 McDonneli St, Amsterdam, N.Y. 12610 Fiied Apr. 1.5, H65, Ser. No. 443,396 6 Claims. (Cl. 333-81) I This invention relates to coaxial devices. More particularly, it relates to improved bilateral resistive coaxial devices which enable advantageous bilateral impedance matching over substantially the entire coaxially-usable frequency spectrum.

Heretofore, a basic problem encountered in known resistive coaxial devices intended for covering a wide frequency range (DC to 11 kmc., for example) or significant portions of such range has been the inherent discrepancy presented between the lumped aspects of the shunt resistive elements which have been utilized in the device and this distributed aspects of the coaxial structure itself. Although prior art structures have enabled the matching of these aspects in a unilateral device, it has not been able to accomplish the matching of these aspects bilaterally.

As is well known in presently used coaxial devices, shunt resistive elements appear as a lumped resistance at all frequencies whereas series resistive elements normally appear distributed since they lie along the center conductor of a coaxial line. At D.C., both the series and shunt resistive elements, of course, appear lumped. However, as frequency increases toward the microwave region, the shunt resistive elements remain lumped but the series resistive elements increasingly become a distributed impedance. Of necessity, therefore, the relationship be- ,tween the series and shunt elements changes as a function of frequency thereby making it impracticable to optimize performance of the device over the entire coaxially usable spectrum.

It is accordingly an important object of this invention to provide a coaxial device in which the coaxial relationship of the shunt resistive elements with the series resistive elements and with the surrounding structure is maintained substantially coaxially constant with frequency.

It is another object to provide a coaxial device in ac cordance with the preceding object which enables bilateral impedance matching over substantially the whole coaxially usable spectrum.

, Generally speaking, the basic concept of the invention is to lump the resistance normally inserted in the center conductor into the shunt resistance which generally is in disc configuration. Thereby, by maintaining the characteristic impedance of the coaxial device (typically 50 ohms) the whole length up to the point of contact with the resistive elements and by having the resistive elements occupy a minimum of electrical length along the coaxial structure, variation in performance of the device in repsonse to frequency variation is greatly reduced.

Accordingly, there is provided a coaxial device comprising a structure adapted to be inserted into electrical contact with a transmission line, comprising a shunt re- 'sistive element and a resistiveelement which is electrically in series, adapted to be lumped physically into the same plane as the aforesaid shunt resistive element, the electrical midpoint of the last named series resistive element being in electrical contact with the shunt resistive element.

For a better understanding of the invention together with other and further objects thereof, reference is had to the following description taken in conjunction with the accompanying drawing its scope is pointed out in the appended claims.

In the drawings:

FIG. 1A is a side elevation, partly in section, of the known bilateral coaxial device inserted in circuit with a coaxial cable;

FIG. 1B is a side elevation, partly in section, of a unilateral coaxial device inserted in circuit with a coaxial cable;

FIG. 1C is a schematic diagram of the electrical circuit provided by the device of FIG. 1A;

FIG. 2A is a side elevation in section of an illustrative embodiment of a bilateral coaxial device constructed in accordance with the principles of the invention inserted in circuit with a coaxial cable;

FIG. 2B is an exploded three dimensional view of the resistive elements included in the device shown in FIG. 2A; and

FIG. 2C is a schematic diagram of the electrical circuit provided by the device of FIG. 2A.

Referring now to FIG. 1A wherein there is shown a presently known bilateral coaxial device 10, the series resistive elements comprise a pair of rods 12 and 14 disposed on the coaxial line of the central conductor 11-? which makes electrical contact with device 10 at an input end 18 and an output end 2i), resistive rod elements 12 and 14 being electrically connected by a coaxially aligned conductive member 22. Also making electrical connection with member 22 and outer conductor 24 is the shunt resistive element 26 which is shown as physically comprising a disc, suitably composed of a dielectric material such as ceramic coated with a resistive material such as carbon. A pair of conductive sleeves 28 and 39 of rough- 1y truncated conical configuration are coaxially disposed with resistive elements 12 and 14 and are chosen, in accordance with known procedures, to have diameters and tapers for providing desired input characteristic impedances. The smaller diameter ends of sleeves 28 and 3d terminate in flanges which make contact with disc 26. The remaining structures of the device of FIG. 1A are the ones usually found in coaxial devices such as the insulation 32 enveloping inner conductor 16 and the outer conductor 24 functioning as a shield, etc.

In FIG. 1B, there is shown unilateral device 34. In this device, since its structure is quite similar to the structure of FIG. 1A elements corresponding to like elements in the structure of FIG. 1A have been given the same numerical designation but with the prime notation. It is seen that the essential difference between the devices of FIGS. 1A and 1B is the shape and disposition of the truncated conical structure 36, i.e., it is larger and its smaller diameter end is located at the output end of the device.

In FIG. 10, wherein there is shown an electrical dia gram corresponding to the structure shown in FIG. 1A, resistances R and R are provided by series resistive rod elements 12 and 14 respectively and shunt resistance R is provided by disc 26. Conductor 40 is provided by truncated conical structures 28 and 30.

Considering the operation of the device of FIG. 1A in conjunction with the schematic diagram of FIG. 1C, under direct current conditions, resistances R 2 R and R are, of. course, all lumped. However, as frequency increases, particularly as it moves progressively into the UMF and microwave regions, series resistances R and R become increasingly distributed impedances while shunt resistance R remains lumped. Consequently, the relationship between the series resistances and shunt resistance necessarily changes as a function of frequency. Because of this phenomenon, optimization of performance of the device over the entirely coaxially unstable spectrum cannot be practicably accomplished.

To illustrate the above deficiency, let it be assumed that the device of FIG. 1A is intended to be used as an attenuator pad which provides an attenuation of 10 db and with desired input and output impedances at ends 18 and to be ohms. In such situation, with resistances R and R chosen to have a value of 25.94 ohms and resistance R to have a value of 35.14 ohms, the required characteristic impedance at point 41 is equal to 24.06 ohms. The impedance Z at point 42 ideally should be equal to 75.94 ohms. However, in the structure of FIG. 1A for it to function bilaterally, the characteristic imepdance of the structure at point 42 is required to be 24.06 ohms.

In the unilateral construction shown in FIG. 10, there is enabled a very good voltage standing wave ratio and input impedance matching at end 18'. However, the matching at output end 20 is poor. With the same values as chosen hereinbefore for the elements of the structure of FIG. 1A, .i.e., 25.94 ohms for resistive elements R and R and 35.14 ohm-s for resistive element R Z at point 42 in FIG. 1C is 75.94 ohms, as is required by the resistor values for input impedance matching.

Prior to describing the structure of FIGS. 2A and 2B, reference is again made to FIGS. 1A and 1B. As has been explained hereinabove in connection therewith, because of their nature, serious coaxial impedance mismatches occur therein. The impedances cannot be matched bilaterally because the aligned series rod resistive elements (which appears as a distributed resistance) and the shunt disc resistance (a purely lumped resistance) do not appear at the same electrical point in the structure. As the applied frequency is increased, the phase difference between the series and shunt resistors is increased.

In FIGS. 2A and 2B, there is shown a bilateral coaxial device constructed in accordance with the principles of the invention in which the resistance, which in prior art devices is normally inserted in the center conductor, is lumped into the shunt resistance which generally has a disc configuration.

In these figures, two discs 72 and 74 suitably composed of a dielectric material such as ceramic, are concentrically disposed, the smaller disc 74 of the two being snugly contained within the larger disc 72. As will be further explained hereinbelow, the smaller disc 74 functions in the same manner as the series resistive rod elements shown in the other figures. In constructing the device of FIGS. 2A and 2B, resistance films are placed on the opposite side 71 and 73 of larger disc 72 and the opposite sides 75 and 77 of smaller disc 74. Conductive metal contact areas 76 and 78 are provided on disc 72 on its outer and inner circumferences respectively. A conductive metal contact area 80 is provided on the outer circumferential perimeter of disc 74 and contact areas are provided at its inner and outer circumferences on sides 75 and 77 such as at 81, but not on its inner circumferential surface. Thus, both sides 71 and 73 of disc 72 are connected by the contact areas. However, in the case of smaller disc 74, both sides are only connected by the contact material on its outer circumferential surface. The contact material may be silver. The resistance coatings provided on the sides of the disc are so chosen that a resistance value of twice the desired shunt resistance is imparted to each side 71 and 73 of larger disc 72.

As shown in FIG. 2A, with smaller disc 74 concentrically snugly disposed in and making electrical contact with larger disc 72, they are electrically connected in a coaxial structure with connections to each side of smaller disc 74 being made with the central conductor 82 enveloped in insulating layer 83 and the outer perimeter of larger disc 72 making electrical connection with the outer coaxial conductor 84. Effectively, the structure of FIGS. 2A and 2B provides two L pads connected together and separated by the width of the discs. The resistances of each paid lie in the same plane. The sizes of discs 72 and 74 and the dielectric material of which they are composed can suitably be chosen to provide the desired characteristic impedance between the surfaces of the disc. Since the impedances can be substantially perfectly matched along the whole frequency spectrum, the attenuator of FIGS. 2A and 2B exhibit very good broadband characteristics, the upper usable frequency being limited only by the cut-off frequency of the TE-1 mode of the coaxial structure.

FIG. 2C shows the electrical circuit provided by the structure of FIGS. 2A and 2B. It is seen therein that with the structure of FIGS. 2A and 2B, the characteristic impedance of the coaxial device (typically 50 ohms) is maintained along the whole length to the point of contact with the resistive elements and by having the resistive elements occupy a minimum of electrical length along the coaxial structure, variation in performance of the device in response to frequency variations is greatly reduced. For a 10 db attenuator design, resistances R and R may each have a value of 25.94 ohms and resistances R and R may each have a value of 70.28 ohms. The impedances Z at the input and output are then 50 ohms and the impedance at points 86 is 36.4 ohms.

Other attenuation values may be constructed by varying the resistor values according to principles well known in the art. Two or more of the aforesaid T pad structurcs can be connected together with a transmission line to obtain a multiple section T pad with increased power handling capabilities.

Although the structure of FIGS. 2A and 2B shows an arrangement in which an entire T pad is deposited on a single disc assembly, it is readily appreciated that individual I. pads can also be constructed in this manner.

Referring to FIGS. 2A and 2C, such an L pad may consist of a resistive coating of finite value on the inner disc at R1 and a restrictive coating of finite value deposited on the outer disc at R3. Surface R4 of the outer disc is non-constructive i.e., has no conductive coating thereon, and surface R2 of the inner disc has a conductive coating (zero ohms). As an example, if an L pad devices is desired to have minimum loss, an input impedance of 50 ohms and an output impedance of 24.06 ohms, then resistor R would have a value of 25.97 ohms, resistor R would have a value of 35.14 ohms, resistor R would be open. i.e., have infinite resistance, and resistor R would have a value of zero ohms. The coaxial structure would be such as to have a characteristic impedance of 50 ohms at the input and 24.06 ohms at the output. By using other appropriate values for R and R such values being derived by method well known in the art, any reasonable combination of input and output impedances can be obtained. Either the higher or lower impedance end of the device could be used as the input. This device could thus be used to match impedances over a very broad band of frequencies.

Two of the aforesaid L pads can be connected together by a transmission line of appropriate impedance to form an attenuator pad of either pi or T configuration, depending on whether the higher impedance sides or the lower impedance sides, respectively, are joined by the transmission line. Such a construction would have higher power dissipation capacity than a single-section device.

The device of FIGS. 2A and 28 can be utilized in a wide variety of resistive coaxial components such as minimum loss pads and resistive power dividers. Because of the excellent impedance match provided by the device of FIGS. 2A and 2B and because of its short coaxial length, the time delay of such coaxial device is short, predictable, reproducible, and, substantially, frequency independent. Such an advantageous time delay characteristic is an important consideration in multiple-channel, balance, or phase-sensitive systems.

While there have been described what are considered to be the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. A coaxial device comprising a structure having an input and output and adapted to be inserted into electrical circuit with a cable having an outer conductor and an inner conductor comprising a shunt resistive element, a resistive element which is adapted to be inserted into series arrangement with said conductor and which is physically disposed to be in substantially co-planar physical disposition with said shunt resistive element, and shunt resistive element comprising a first disc resistive element having a circular opening therethrough, an electrically conductive outer circumferential perimeter adapted to make electrical contact with said outer conductor and an electrically conductive inner circumferential perimeter, a second resistive disc element having an opening therethrough and an outer circumferential perimeter equal to the inner circumferential perimeter of said first disc and a width substantially equal to the width of said first disc, said second disc being disposed within the opening of said first disc with its sides substantially continuous with the sides of said first disc, and electrical contact areas on each side of said second disc along its outer and inner circumferences respectively which are adapted for making contact with said inner conductor.

2. A coaxial device as defined in claim 1 wherein said first disc resistive element comprises a dielectric material having resistance films of chosen respective values on each side thereof and a coating of electrically conductive material on its outer and inner circumferential surfaces and said second disc resistive element comprises a dielectric material having resistance films of selected respective values on each side thereof, and contact areas of electrically conductive material on each side thereof along its inner and outer circumferences, said chosen and selected respective values providing given operating characteristics.

'5. A coaxial device as defined in claim 1 wherein said first disc resistive element comprises a dielectric material having .a resistance film of a given finite value on one side and a non-coated substantially non-conductive other side, and wherein said second disc resistive element has a resistance film of a selected finite value on one side and a substantially conductive coating on its other side, said first and second elements being disposed to effect co-planar disposition of the respective resistance film coated sides thereof, the values of said resistance films being chosen to provide an L pad.

4. A coaxial device as defined in claim 2 wherein said respective values of the resistance films on each side of said first disc resistive element are each chosen tobe about equal to twice the value of the shunt resistance of said device and wherein the respective values of the resistance films on each side of said second disc reisistive element are each selected to be about equal to one-half the value of the total series resistance of the device between said input and said output, said values of said resistance films being chosen to provide a T pad.

5. A coaxial device has defined in claim 3 and further including a further plurality of like coaxial devices, and a plurality of coaxial line sections for respectively connecting one of said devices to another of said devices, each of said coaxial line sections having a characteristic impedance whose value is chosen in accordance with said resistance values and predetermined operating characteristics.

6. A coaxial device as defined in claim 4 and further including a further plurality of like coaxial devices, and a plurality of coaxial line sections for respectively connecting one of said devices to another of said devices, each of said coaxial line sections having a characteristic impedance whose value is chosen in accordance with said resistance values and predetermined ope-rating characteristics.

References Cited by the Examiner UNITED STATES PATENTS 2,620,396 12/1952 Johnson et a1. 333-81 2,831,163 4/1958 Stevens 33322 2,968,774 1/1961 Rodriguez 3 3381 HERMAN KARL SAALBACH, Primary Examiner. R. F. HUNT, JR., Assistant Examiner. 

1. A COAXIAL DEVICE COMPRISING A STRUCTURE HAVING AN INPUT AND OUTPUT AND ADAPTED TO BE INSERTED INTO ELECTRICAL CIRCUIT WITH A CABLE HAVING AN OUTER CONDUCTOR AND AN INNER CONDUCTOR COMPRISING A SHUNT RESISTIVE ELEMENT, A RESISTIVE ELEMENT WHICH IS ADAPTED TO BE INSERTED INTO SERIES ARRANGEMENT WITH SAID CONDUCTOR AND WHICH IS PHYSICALLY DISPOSED TO BE IN SUBSTANTIALLY CO-PLANAR PHYSICAL DISPOSITION WITH SAID SHUNT RESISTIVE ELEMENT, AND SHUNT RESISTIVE ELEMENT COMPRISING A FIRST DISC RESISTIVE ELEMENT HAVING A CIRCULAR OPENING THERETHROUGH, AN ELECTRICALLY CONDUCTIVE OUTER CIRCUMFERENTIAL PERIMETER ADAPTED TO MAKE ELECTRICAL CONTACT WITH SAID OUTER CONDUCTOR AND AN ELECTRICALLY CONDUCTIVE INNER CIRCUMFERENTIAL PERIMETER, A SECOND RESISTIVE DISC ELEMENT HAVING AN OPENING THERETHROUGH AND AN OUTER CIRCUMFERENTIAL PERIMETER EQUAL TO THE INNER CIRCUMFERENTIAL PERIMETER OF SAID FIRST DISC AND A WIDTH SUBSTANTIALLY EQUAL TO THE WIDTH OF SAID FIRST DISC, SAID SECOND DISC BEING DISPOSED WITHIN THE OPENING OF SAID FIRST DISC WITH ITS SIDES SUBSTANTIALLY CONTINUOUS WITH THE SIDES OF SAID FIRST DISC, AND ELECTRICAL CONTACT AREAS ON EACH SIDE OF SAID SECOND DISC ALONG ITS OUTER AND INNER CIRCUMFERENCES RESPECTIVELY WHICH ARE ADAPTED FOR MAKING CONTACT WITH SAID INNER CONDUCTOR. 